We shall test the validity of our hypothesis (Fan and Rodwell 1974b) that intracellular levels of coenzymes regulate degradation by intracellular proteases of certain intracellular enzymes. Initially, we shall study alpha-ketoarginine decarboxylase, a thiamine pyrophosphate (TPP) protein, as the experimental system. Later, we plan to extend our observations to other TPP-enzymes (pyruvate and alpha-ketoglutarate decarboxylases; transketolase) and subsequently to enzymes which require other coenzymes (FAD, NAD, B6-phosphate). We hope thus to validate a general model for regulation of protein degradation in bacteria in which various ligands (coenzymes, substrates, metals) stabilize enzymes towards proteolytic degradation. A prominent feature of our model is that we do not postulate changes in proteolytic enzyme activity to account for changes in rates of protein degradation. We propose instead a change in susceptibility of proteins to proteolytic attack. This increased susceptibility involves dissociation of a ligand such as a coenzyme. Questions we shall ask include: 1. Do intracellular levels of ATP, TPP, enzyme and apo-enzyme vary during growth and starvation as predicted by our model? 2. Does loss of alpha-ketoarginine decarboxylase reflect activity of a general or specific protease? 3. How general is the observation that TPP stabilizes enzymes that require it towards proteolytic degradation? 4. Can these observations be extended to enzymes which require other coenzymes? Which require cofactors such as metals? 5. Can the degradation of cofactor-requiring enzymes be accelerated by starving the cell for a specific coenzyme?